RTV process done correctly in receiving but Vendor not set up to create auto credit memos

We have some returns on PO's that were done using the Return to Vendor option

Unfortunately the vendor for the po, was not set up to auto create credit memos.

So now we have PO lines that look as if we overpaid them:

QTY ORDERD   100

QTY RETURNED 50

TOTAL RECEIVED QTY 50

TOTAL PAID QTY 100

how do i reduce the AP side of the transaction ?

Things to note - we took in cash for some of these (Vendor sent us a check for returned items) but since we didn't know how to do that debit memo after the fact, it was just applied to Purchasing GL not to the Vendor against any invoices...